Fanotec Ninja 4 User Manual - page 23
© 2005-2011 Nodal Ninja
®
- 23 -
Finding the Entrance Pupil of a Lens
What is the Nodal Point or Entrance Pupil and why is it important?
We would like to take moment and dispel
misconceptions of the word “Nodal Point”.
When referring to this no parallax point
(NPP) many still refer to it as the Nodal
Point. Technically the point at which you
would rotate the camera/lens to avoid
parallax is called the “entrance pupil”.
The entrance pupil is a floating point
located inside the lens at the point where
the light refracts or reverses itself before
continuing to the image sensor or film
plane. The entrance pupil is different on
each lens and changes at different focal
lengths.
The entrance pupil (incorrectly referred to by some as nodal point) of a lens is the virtual
image of the aperture formed by the lens elements in front of it, and is the place where
lightpathscrossbeforebeingfocusedontotheCMOS,CCDorfilmplane.Foralightray
to pass through the lens, it must be directed towards the entrance pupil, which is therefore
the center of perspective. Note that the entrance pupil can be outside of the lens and even
behindthefilmplane!Whentakingadjacentimagesyouwanttorotatethecameraaround
alinethatrunsthrough(orverycloseto)theentrancepupil.Byfindingtheentrancepupil
of the lens, and rotating the camera about this point, you will assure parallax free images.
Parallaxismoreobviousintheviewfinderwithobjectsclosetothecamera.Themoredistant
the objects the lesser the parallax. If using a lens with zoom capability the entrance pupil will
change in position as the zoom changes. Because of so many variables, it is best to learn
howtofindtheentrancepupilmanually.